This Tech Note will discuss Lead / Lag phasing engineering considerations when running non-coordinated, coordinated and Synchro Green intersections and corridors.  This document references the FHWA Traffic Signal Timing Manual (FHWA-HOP-08-024) and the Transportation Research Record 1324: Guidelines for Use of Leading and Lagging Left-Turn Signal Phasing.

Phasing and Lead / Lag left turn Phasing Overview

Most traffic signals apply STD8 operation even if all eight phases are not enabled. NEMA assigns the left-turn movements to the odd-numbered phases and the through movements to the even numbered phases. It is easy to remember this convention if you recall that the even numbered through phases are assigned in a clockwise manner (2-4-6-8) and the left-turn phases opposing each thru are numbered in pairs 1-2, 3-4, 5-6 and 7-8. Many agencies assign phase 1-2-5-6 to the major (coordinated) street and 3-4-7-8 to the cross street as shown below. Other agencies assign phases to a direction (north, south, east or west) if the non-intersecting streets in the network are parallel.

When operating an intersection, the agency must consider all movements of traffic including left turn movements.  Intersection layouts are based on many factors including geometry, user characteristics (trucks, cars pedestrians), user demand, measured volumes, capacity and critical movement analysis.

Left turns at intersections have long been a source of concern for traffic engineers. In recent years, greater traffic volumes at many intersections and fiscal and right-of-way constraints on construction have led traffic engineers to design and implement increasingly sophisticated signal schemes to allow

vehicles to turn left safely and efficiently.

Left turns can be protected or permissive.  Protected left turns can be leading or lagging the thru traffic as shown below.

The permissive scheme is the most common type of signal scheme accommodating left turns in the United States. In this scheme, vehicles may turn left when receiving the green-ball signal and when sufficient gaps appear in the opposing traffic stream, which also has a green-ball signal. In another very common signal scheme, the protected scheme, vehicles may turn left only when receiving a green-arrow signal, which affords them exclusive right-of-way through the intersection. In most applications, the protected signal is given to vehicles turning left before the green ball is given to the through movement on the same street (i.e., protected leading). In most applications, the protected signal is given to vehicles turning left before the green ball is given to the through movement on the same street (i.e., protected leading). Most other common signal schemes to accommodate left-turning vehicles involve a variation on or combination of permissive and protected schemes, including:

• Protected-lagging, in which the green arrow is given to left-turning vehicles after the through movements have been serviced;

• Protected-permissive, in which protected left turns are made first in the cycle and a green-ball signal allows permissive left turns later in the cycle; and

•Permissive-protected, in which permissive left turns are allowed first in the cycle and protected left turns are accommodated later in the cycle.

Based on the needs of the agency, the engineer can choose when they want left turns to run. In standard 8 phase mode there are 16 different sequences, that can be chosen to satisfy all lead and lag left turn combinations.

Many agencies choose to use a flashing yellow arrow, for left turns, to warn and control the permissive left.

Once an agency determines how to set up their intersections, they may want to operate them in free or coordinated modes. Lead /Lag lefts are normally engineered for the best traffic flow during Free operation (fixed recalls or actuated detection).  When a user operates signals in coordination they may use lead/lag phasing to improve green band efficiency.  In addition, because SynchroGreen adaptive uses coordination as its basis, it could also be impacted.    

Controller Software Decision Considerations

Once an agency chooses how to set up their intersections, they may want to operate them in free or coordinated modes. As explained above setting up coordination creates an additional layer of engineering.

Coordination Parameter Calculations

When an agency sets up Coordination for a set of intersections, the controller software will calculate 

how to split up the cycle for each phase movement.  The user can review these calculations using the Easy Calcs screen at MM-2-8-2.

All that is required to allocate cycle time using NTCIP FIXED and FLOAT are the Split Times (in seconds) for each phase. The controller automatically calculates the internal force-off and yield points (called Easy Calcs) given the split times and sequence of the pattern. However, for most users, the Easy Calcs (force-off and yield points calculated under FIXED and FLOAT) are “hidden from view” and all the user is concerned about is ensuring that the split times provided pass the coord diagnostic. The Split Table above assigns phase 2 as the Coordinated Phase with 25” Split Times allocated to each phase. 

The pattern example above represents a 100” cycle with the offset referenced to Begin-of-Green (BegGRN) coord Ø2. All splits are 25” as shown in the Split Table# above and the clearance times for each phase are 5”. The zero point of the cycle (Loc = 0) coincides with the beginning of the coordinated phase (in this case, phase 2).  The green interval for Ø2 (and Ø6) is starts at Loc=0 and is ended at Loc=20 to provide a 25” Split Time once the 5 second clearance occurs. Each phase in the sequence is forced off 25” after the force-off for the previous phase starting at the coord phase and proceeding across the barriers. 

The Easy Calcs status screen (MM->2->8->2) displays the internal calculations for this example under FIXED or FLOAT NTCIP modes.  Secondary Force-offs only apply to the OTHER modes, so under FIXED and FLOAT, the Primary and Secondary Force-offs are the same. The Yield points opens the Permissive Periods to service vehicle and pedestrian calls for each phase. The Apply points close the Permissive Periods.  

If the user changes pattern lengths or offsets, the software must get in step with the new pattern.  For example, if the agency desires that the pattern length change from 90 sec to 120, the controller will need to smooth its way and add 40 seconds to the cycle length.  If the agency desires to remain in the artery for until its force off (i.e. setting Return Hold to ON under MM->2->5) the new force off may keep the controller in a phase longer than the previous pattern depending on when the transition occurs.

Sequence changes will also affect the way the software will get in step with the new pattern.  If phase rotations occur, then the controller software will need to get in step with the new rotation as well as recalculate cycle parameters based on the new rotation. It is recommended that the user set the parameter FreeOnSeqChg to ON so the controller will change the sequence at a barrier as well as recalculate cycle parameters based on the new cycle length and phase rotation.

Below is a summary of the Easy Calcs parameters.

Primary Force-Off

The Primary Force-Off is the point in the local cycle that a force-off is applied to a phase causing that phase to terminate and begin timing yellow clearance.  A Primary Force-off will remain applied until the phase terminates.

Secondary Force-Off

The Secondary Force-Off is a momentary force-off applied prior to the Primary Force-off.  Secondary Force-offs are useful when conditionally servicing phases or when a phase is to be forced off twice per cycle.  The Secondary Force-off normally default to the value of Primary Force-off.  NOTE: This feature is not used in NTCIP Coordination.

Vehicle Yield

The Vehicle Yield is that point in the cycle that a vehicle call on a phase will be serviced, i.e. that the phase’s inhibit is removed.  Note that the phase inhibit is automatically applied by the controller at a calculated time in advance of the primary force-off.  

Vehicle Apply

Vehicle Apply Point (VehAply) = Primary Force-off – ((Max Yellow + All Red ) + Minimum Green)

The yield point must be earlier than the automatic application point for the phase to be serviced.  If short-cycle offset correction is enabled, the yield point must be earlier still to allow for the effective reduction in split time that occurs when the local cycle timer corrects by running fast.

Pedestrian Yield

The Pedestrian Yield is that point in the cycle that a pedestrian call on a phase will be serviced, i.e. that the phases pedestrian inhibit is removed.  The phase inhibit is automatically applied by the controller at a calculated time in advance of the primary force-off. 

Ped Apply

The Ped Apply point defines the point in the cycle when the pedestrian phase inhibit is applied. A pedestrian phase may begin anytime between the Ped Yield point and the Ped Apply point. The PedApply point for each pedestrian phase is calculated as:

   Ped Apply Point (PedAply) = Primary Force-off – ((Max Yellow + All Red) + Pedestrian Clear)

The same considerations described above for selecting vehicle yield points apply to determining pedestrian yield points except when the STOP-IN-WALK is enabled.  Refer to the explanation of Stop-In-Walk.

FloatMx

Floating max time (FloatMx) is equal to the green portion of the split needed to terminate the phase prior to the force-off if the time allocated to the phase exceeds programmed split time. This is used as the max green time with floating force-offs.

PedLeav

The Pedestrian Leave Point is used when Rest-In-Walk is active. This is the point in time when the Pedestrian Clearance begins after the phase has been resting in walk.

PedCall

Ped Call displays the last time a call can be placed in the cycle so a pedestrian can be serviced in that cycle. Ped Call is only used when MinP is active, otherwise Ped Call = Ped Apply. The Ped Call point for each pedestrian phase is calculated as:

     PedCall = Ped Apply - Max (red+yellow)

SplitRem

This is the remaining time in the split before the next cycle begins.

Lead Lag Calculation Examples – Choosing the coordinated Phase

This section will show the effect on the Easy Calcs when lead lag left sequencing occurs.  In this section we will change Lead/ Lag phasing to show the effect on easy calcs

Consider STD8 Phase with the following programming: 

The coordination will run Pattern 1 (ENDGRN) as setup below

The Easy Calcs created when running this pattern as shown below:

Choosing Phases 2 or 6 as the coord Phase would have no impact because both lefts are leading. 

Note: NTCIP requires that only one phase is chosen as the coordinated phase.  In this case we chose phase 2.   Phase 6 which runs with phase 2 gets the benefit of coordination and is also known as the “Pseudo-Coord” phase.

Now change the phase sequence to Sequence # 4 to make Phases 1 and 5 Lagging Phases:

The Easy Calcs are shown below:

Choosing Phases 2 or 6 as the coord Phase would have no impact because both lefts are lagging.

Now choose Phase 1 as a Lagging left and Phase 5 as a leading Left using sequence 3.

The Easy calcs are shown below

Phase 5 leaves before the “ENDGRN” for Phase 2.  This in effect, gives Phase 6 an extra 10 seconds of green, which may be unintended.   This could lead to inefficiency. In addition, if other phases in the corridor have sequences, unintended queues on those intersections may occur.  Placing the coord reference point between phases (in this case at Phase 2) and not at a barrier is allowed, but the agency should intend to do so. 

A recommended and simple way to resolve this issue is to change the coordinated phase to the phase that controls coordination AT THE BARRIER, in this case Phase 6.  

Below is the Easy Calcs when phase 6 is the coord Phase.

Similar results occur if the Coord Reference Point is changed to BEGGRN.

For Sequence 1, The Easy Calcs are shown below.

Choosing Phases 2 or 6 as the coord Phase would have no impact because both lefts are leading. 

Now change the phase sequence to Sequence # 4 to make Phases 1 and 5 Lagging Phases:

The Easy Calcs are shown below:

Choosing Phases 2 or 6 as the coord Phase would have no impact because both lefts are lagging.

Now choose Phase 1 as a Lagging left and Phase 5 as a leading Left using sequence 3 with Phase 6 as the Coordinated Phase 

The Easy calcs are shown below

Note that Phase 2 (the Pseudo-Coordinated Phase) bridges between the last cycle and the new cycle.  In fact, once the next cycle begins phase 2 only has 5 seconds of green begore it clears to run Phase 1. This could have un intended results.
 

Instead if we choose 0Phase 2 as the Coordinated Phase under the same Sequence 3 Lead/Lag Left scenario. 

The Easy calcs are shown below

Although the Lead/Lag lefts occur, it has no effect on coordination or phase splits.

To repeat, a recommended and simple way to resolve this issue is to change the coordinated phase to the phase that controls coordination AT THE BARRIER, in this case Phase 2.  

SynchroGreen Adaptive Considerations

The fundamental difference between SynchroGreen and traditional TBC is that SynchroGreen transmits new cycle, split and offset data each cycle.  At the local zero, the controller uses that data to recalculate new Easy Calcs, which control how the signal times during the cycle.  

The preferred phasing is Standard 8 Phase operations (STD-8) or Quad Sequential (QSeq) operations.  In the event that Standard 8 Phase operation cannot be enabled due to non-standard phasing, User Programmable Mode (USER) should be used.  If USER mode is selected, the agency should verify that all ring sequences used for adaptive operations all form complete ring and barrier structures.  Incomplete ring and barrier structure may produce errors in SynchroGreen.  Incomplete ring and barrier structures will commonly result in coordination errors.  It is a good rule of thumb to include all eight phases, (e.g., 1, 2, 3, 4, 5, 6, 7, 8) in the ring and barrier structure even if a phase is not enabled.

A difference between ring and phasing structures between controllers operating standard coordination and adaptive coordination is that controllers operating adaptively must have an enabled phase in each ring in each barrier group with phase times.  As an example, consider the following intersection.   Phases 1,2,3,4,5,6 are active at this intersection.  To pass coordination checks, the user must define time in for enabled phases 1-6 as well as time for inactive phases 7 or 8, as shown below.  This applies when programming standard or adaptive coordination plans.    However, when programming adaptive plans, the user must enable phase 8 as a “dummy” or “ghost” phase in MM->1->1->2 and define phase times in MM->1->1->1.  A standard practice is to define a minimum green of 1” in the dummy phase 8.  

The above only applies when operating a controller in STD-8 or USER phase mode.  If a controller is operated in QSeq phase mode, the dummy phase programming previously shown is not required

Another difference between defining standard coordination patterns and adaptive coordination patterns is that the for a SynchroGreen pattern must be “touching a barrier.” In other words, when the Offset Reference Point is at the beginning of green, it is important that the coordinated phase be the leading phase.  Alternatively, when the Offset Reference Point is at the end of green (beginning of yellow), the coordinated phase should be the lagging phase.   This is important so that Synchro-Green can provide consistent and predictable coordination.  The user should check the phase sequence (MM->1->2->4), phase concurrency tables (MM->1->1->4) and the reference points (MM->2->5) to verify that adaptive patterns are properly configured.  This is shown below in the following examples:

Summary

Lead Lag Considerations have an impact on signal operations and the engineer must evaluate the implications of the lead Lag phasing.  Once engineered the agency should ALWAYS test the operations prior to installing in the field.